Can someone please tell me what RPO codes I'm looking for to find what I have in my rear? I'd like to know gear ratio and if I have posi or not. Also, if I have posi, what gear oil am I supposed to be using? I've been told it's different than for a regular diff. Thanks

Just a pure guess...You have 2:73 open rear with a 700R4 Auto and a 305 Carb/TBI and it is a base model Camaro/Firebird. Reason because the largest numbers were made with the above.

Most of the rear end codes start with a G. The posi code is G80. Gear ratios are GT4-3.73, GU6-3.42, GU5-3.23, GU4-3.08,. It's been a while since I looked at the codes, but I think there is a standard code that starts with a Q that doesn't give you any more information. This code just means that you have the standard gear for the engine and transmission that you have.

And, of course, if the rear end isn't original to the car, the RPO's mean nothing.

The lube probably hasn't been changed in a decade or more, so jerk the diff cover and have a peek. Here's how to determine what ratio the rear is: numbers are stamped on the ring and pinion:

41 : 15 = 2.73
41 : 14 = 2.93
40 : 13 = 3.08
42 : 13 = 3.23
41 : 12 = 3.42
41 : 11 = 3.73

You'll also be able to take note of whether the rear has a limited slip. If it has, use GM's limited slip additive along with a suitable gear lube. I use Mobil 1 75-90W.

JamesC

I forgot to add the number for the GM oil additive. 1052358 is the GM number. Use 75W90 or 80W90 gear oil. Usually 2 quarts, plus the additive, will do it.

I found GU4 and G80 in the list.

So I take it thats a 3.08 Posi right??

What does this special additive for posi do? What if I don't use it?

Also why isn't my signature showing up???

The additive allows the clutch in the posi to slip when you turn a corner. If you don't use the additive then the rear end will probably pop and chatter when you turn. I usually recommend that people who drag race and do water burnouts not use the additive, or not as much of it.